A color video signal is typically displayed as superimposed red, green and blue signals. The red, green and blue signals are typically derived by means of a linear transformation from luminance (Y) and chrominance (U and V) components.
Some formats of professional video tapes store the component Y, U, and V digital signals separately. Also, commonly used standards for uncompressed digital video (e.g., CCIR 601) and compressed digital video (e.g., JPEG, MPEG 1 and MPEG 2, DVC) also represent the components separately. The separate digital components may be time-multiplexed, and may share some auxiliary information (e.g., time codes and closed-caption information for uncompressed video, and motion vectors or quantisation matrices for compressed video), but the signals themselves are independent; some bits are used for the Y signal, some for the U signal, and some for the V signal.
Video may also be represented in composite form; e.g., NTSC or PAL. A composite video signal is generated by quadrature modulating the chrominance components and then adding them to the luminance component. The result is called a composite video signal. Composite video signals are used in (analogue) broadcast and professional and home video tapes. A discussion of composite video encoding (converting component video into composite video) and decoding (converting composite video into component video) can be found in "A Technical Introduction To Digital Video", Charles A. Poyton, John Wiley & Sons, Inc. 1996. Such a discussion can also be found in "Digital Video; An Introduction to MPEG-2", Barry Haskell, et al. Chapman & Hall, 1997.
To convert a composite video signal into a compressed digital video signal, it is composite decoded (to obtain separate Y, U and V signals). The individual component signals are then compressed using, for example MPEG-1 or MPEG-2 or motion JPEG. The conversion from analog to digital can occur before or after composite decode.
Composite Encoder
A simplified block diagram of a composite encoder is shown in FIG. 1. The encoder 10 receives the component signals, Y, U, V as its inputs on lines 7, 8, and 9. The input chroma signals U and V are low-pass filtered using the low-pass filters 12 and 14. The filter outputs are then quadrature modulated using the modulators 16 and 18 with a subcarrier at frequency Wsc. ##EQU1##
The quadrature modulated chrominance components are then added to the luma signal Y by two adders 19 and 20. The output on line 21 is a composite video signal. The subcarrier modulation frequency Fsc is an odd multiple of half the line frequency, Fl (the number of lines of video per unit time); i.e., EQU Fsc=(m+1/2)Fl (2)
when m is an integer.
For example, in the case of the NTSC video format, Fl is 2250/143 kHz=15.73 khz and Fsc is 227.5 Fl=315/88 Mhz=3.58 Mhz.
The in-phase carrier cos(Wsc.multidot.t) and the quadrature phase carrier sin (Wsc.multidot.t) are generated by the subcarrier generator 22. The filtered U component is modulated onto the in-phase carrier. The filtered V component is modulated onto the quadrature phase carrier.
Composite Decoder
A conventional composite video decoder 30 is shown in FIG. 2. The input is a composite video signal on line 31. The composite video is sent to two vertical comb filters 32, and 34; a luma comb filter 32 produces the output luma signal Y and a chroma comb filter 34 produces the quadrature modulated chroma components. A subcarrier regenerator 36 is then used to regenerate the chroma subcarrier. The modulated chrominance components are demodulated by multiplying by the regenerated chroma subcarrier and low-pass filtering. Specifically, the output of the chroma comb filter 34 is multiplied by the in-phase carrier cos(Wsc.multidot.t) by multiplier 38 and horizontal low-pass filtered by filter 39 to obtain the U component. The output of the chroma comb filter 34 is multiplied by the quadrature phase carrier sin(Wsc.multidot.t) using the multiplier 40 and horizontal low-pass filtered by filter 41 to obtain the V component.
Digital Compressor
A conventional video pre-processor and compressor 50 is shown in FIG. 3. The input signals are the Y, U, and V components in digital form. Each input component signal is vertically low-pass filtered and re-sampled using the filters, 51, 52 and 53. It should be noted that filtering and re-sampling is not utilized in all cases. For example, in 4:2:0 MPEG-2, the luminance component is not re-sampled vertically, but the chroma components are re-sampled vertically. The luma component is typically vertically filtered for low bitrate (less than 4 mbit/sec) compression but not for high bit rate compression. The filtered and re-sampled component video undergoes additional pre-processing using the pre-processor 55 and is then compressed using the digital compressor engine 57. Illustratively, the compression engine is an MPEG-2 video compressor. In this case, the additional preprocessing may involve temporal nonlinear filtering and obtaining certain statistics relating to the frames to help the compression engine code more efficiently and may also involve frame reordering if B frames are involved.
Composite Video Decoder and Compressor
A conventional composite video decoder, pre-processor and compressor is shown in FIG. 4. A conventional composite video decoder 60 decodes the composite video (in the manner described above in connection with FIG. 2) and the component output (Y, U, V) is sent to a conventional video pre-processor and compressor 70. The pre-processor includes the filters 51, 52, 53, as well as the additional pre-processing capabilities 55. Illustratively, the compressor is an MPEG-2 compressor. The output on line 71 is an MPEG-2 bit stream in this case.
In short, according to the prior art, a composite video signal is decoded:
(1) by using a luma comb filter to obtain the luma (Y) component and then vertically filtering and (optionally) re-sampling to obtain a filtered and re-sampled luma component; PA1 (2) by using a chroma comb filter to obtain quadrature modulated chroma components, demodulating the chroma component to obtain separated U and V chroma components, and then vertically filtering and re-sampling the separate chroma components to obtain vertically filtered and re-sampled chroma components.
After this, the filtered and optionally re-sampled luma component and the filtered and resampled chroma components can be compressed using for example an MPEG-2 compressor.